Sensor with at least one micromechanical structure and method for production thereof

ABSTRACT

The invention relates to a sensor with at least one silicon-based micromechanical structure, which is integrated with a sensor chamber of a foundation wafer, and with at least one covering that covers the foundation wafer in the region of the sensor chamber, and to a method for producing a sensor.  
     It is provided that in the sensor of the invention, the covering ( 13 ) comprises a first layer ( 32 ) (deposition layer) that is permeable to an etching medium and the reaction products, and a hermetically sealing second layer ( 34 ) (sealing layer) located above it, and that in the method of the invention, at least the sensor chamber ( 28 ) present in the foundation wafer ( 11 ) after the establishment of the structure ( 26 ) is filled with an oxide ( 30 ), in particular CVD oxide or porous oxide; the sensor chamber ( 28 ) is covered by a first layer ( 32 ) (deposition layer), in particular of polysilicon, that is transparent to an etching medium and the reaction products or is retroactively made transparent; the oxide ( 30 ) in the sensor chamber ( 28 ) is removed through the deposition layer ( 32 ) with the etching medium; and next, a second layer ( 34 ) (sealing layer), in particular of metal or an insulator, is applied to the deposition layer ( 32 ) and hermetically seals off the sensor chamber ( 28 ).

[0001] The invention relates to a sensor with at least one silicon-basedmicromechanical structure, having the characteristics recited in thepreamble to claim 1, and to a method for producing such a sensor, havingthe characteristics recited in the preamble to claim 18.

PRIOR ART

[0002] Sensors that have silicon-based micromechanical structures areknown. If the micromechanical structure is movable element (sensorelement), then such sensors can be used as acceleration sensors, rotaryacceleration sensors, inclination sensors, resonant magnetic fieldsensors, or rotation rate sensors. Typically, these sensors comprise afoundation wafer, which is usually likewise formed from material thatcontains silicon, in which the structure is integrated into a so-calledsensor chamber of its surface. To protect the structures and theatmosphere prevailing in the sensor chamber, the foundation wafer iscovered with a cap wafer, with a covering that covers at least thesensor chamber. This cap wafer, because of its micromechanicalprestructuring, has many individual caps joined together, of which eachindividual cap comes to rest exactly above the sensor chambers and issoldered to the sensor chamber in hermetically sealed fashion, and thushermetically seals off the underlying sensor structure from theenvironment.

[0003] From German Patent Disclosure DE 195 37 814 A1, the production ofsuch sensors is known. Based on a silicon substrate, insulation layersand conductive layers (in the form of electrodes or electricalconnections) are applied in alternation, using the conventional methodsteps known from semiconductor technology. By means of masking andmachining methods, also known, structuring of such layers can be done,for instance by way of lithography or etching processes. In an ensuingprocess step, a polycrystalline silicon layer (epipolysilicon), with alayer thickness ranging from a few nanometers to several tens ofmicrometers, preferably from 10 to 20 μm, is created. From this siliconlayer, in the final analysis the required structures are etched out andmade freely movable by underetching. The previously applied, structured,buried conduction layer makes it possible to establish electricalconnections between elements of the sensors and the “outside world”, inthe form of so-called connection regions. These connection regions,which are connected via the conductive layer to sensor elements, carry ametallizing on their surface. The connection region with the metallizingapplied over it serves to secure bonding wires, with which an electricalcontact with the structures in the sensor chamber (sensor structure) isthen to be established. The sensor structure described in DE 195 37 814A1 is distinguished by the fact that it has a movable (free-standing)region with measurement capacitors, where changes in the measuringcapacitance upon a deflection are used as a measurement variable.

[0004] The components of the sensor, all described as examples here,will simply be called the foundation wafer, for the sake of simplifyingthe further description. The foundation wafer must be hermeticallytightly joined to the cap wafer in a final machining step. To that end,in the prior art it is provided that a cap be secured above each sensorchamber to the surface of the foundation wafer by means of a glasssolder layer on the cap wafer (known as the seal-glass solder process).A disadvantage of this is that this technology is relatively expensive.The glass solder layer must be applied to the micromechanicallystructured cap wafer by means of screen printing. The cap wafer mustalready be structured on both sides to enable the ensuing covering andcontacting of the sensor; that is, the cap wafer itself is alreadyintrinsically expensive. Moreover, this capping technique requires arelatively large amount of space, in which up to about 75% of theindividual element area is required for anchoring the cap to the sensorchip. The resultant structural height and limited structuring optionspreclude the use of certain especially economical housings for thesensor.

[0005] Often, the free-standing regions covered by the caps of the capwafer are relatively large. Sensor structures often have edge lengths ofseveral hundred micrometers. If such a sensor is subjected to amechanical overload, then in an extreme case, sagging of the cover layercan lead not only to interference with the sensor properties but in thefinal analysis also to an excessive deflection of the sensor structure,to the point of irreversible damage.

ADVANTAGES TO THE INVENTION

[0006] According to the invention, the disadvantages of the prior artare overcome by the sensor and the method for producing the sensorhaving the characteristics recited in claims 1 and 18, respectively.Because in that the covering comprises a first layer (deposition layer)that is permeable to an etching medium and the reaction products, and ahermetically sealing second layer (sealing layer) located above it, itis possible in terms of process technology to dispense with theexpensive cap wafer, the conventional screen printing and solderingmethods, and the large reserve areas for glass solder technology, andthus the processing can be completed substantially less expensivelyoverall. Because

[0007] (a) at least the sensor chamber present in the foundation waferafter the establishment of the structure is filled with an oxide, inparticular CVD oxide or porous oxide;

[0008] (b) the sensor chamber is covered by a first layer (depositionlayer), in particular of polysilicon, that is transparent to an etchingmedium and the reaction products or is retroactively made transparent;

[0009] (c) the oxide in the sensor chamber is removed through thedeposition layer with the etching medium; and

[0010] (d) next, a second layer (sealing layer), in particular of metalor an insulator, is applied to the deposition layer and hermeticallyseals off the sensor chamber,

[0011] it is possible to enable structuring of the coveringretroactively, via the masking and machining methods known fromsemiconductor technology.

[0012] By steps (a) and/or (b), planarizing of the wafer surface can beperformed (for example by CMP, for Chemo-Mechanical Polishing). Existingprocessing problems resulting especially from topographies, such asapplying and structuring the bond pads (metallizing), are thuscircumvented. Compared to the established cap process, the simplifiedcourse of the process also produces markedly reduced production costs.The invention thus provides access to a sensor of the generic type anddiscloses a method for producing the sensor, by which it is possible forthe first time to employ capping with a markedly lower structural heightfor hermetically sealing off sensor chambers in micromechanicalstructures, so that it is now possible to install them in theaforementioned, especially economical housings.

[0013] The permeability of the deposition layer for the requisiteetching medium and the reaction products produced during the etching canbe forced in two different ways. First, by anisotropic etching, etchingopenings can be made in the deposition layer, for instance by thesilicon deep etching process patented in German Patent DE 42 41 045. Thesize and location of such etching openings can be defined in a verytargeted way photolithographically by masking, so that it is possibleamong other things to keep any later exposure of the sensor chamber tothe hermetically sealing material forming the sealing layer as slight aspossible. It is possible to create etching openings that have a diameterof fractions of micrometers up to several micrometers and that can besealed off in a relatively short time, in a manner to be describedbelow. This is attained for instance by means of a high aspectratio—that is, a ratio between the depth and diameter of the etchingopenings.

[0014] Second, for the covering, permeable materials such as silicon,polysilicon or epipolysilicon, which is already permeable because of thedeposition conditions or is made permeable by subsequent processing, canbe used, at least in some regions.

[0015] An advantageous method for forcing the transparency of thecovering is to employ electrochemical etching operations. One suchmodification of the deposition layer is done in a suitable electrolyte,such as a mixture of hydrofluoric acid and ethanol. The silicon of thecovering exposed to the etching operation is converted in this processinto porous silicon, or in other words it is made porous. Regions of thedeposition layer that are not to be made porous can be protected in aknown manner by means of masking layers or suitable doping (such as n⁻).An electrical connection by applying an anodic potential could be doneboth via the top side and the underside of the deposition layer. In thelatter case, the anodic potential is applied to the layer ofepipolysilicon located below the deposition layer and forming both thematerial comprising the sensor structures and the bond frame of thefoundation wafer. It is advantageous in this respect that the bond framecan be joined directly electrically to the foundation wafer. Anadditional electrical connection between the foundation wafer and thecovering exists in the form of support elements, which can be providedfor the sake of mechanically stabilizing the covering. It is thuspossible in a simple way to perform the electrical contacting of thedeposition layer via the foundation wafer from the back side of thefoundation wafer (back-side contact).

[0016] The etching operation can be additionally reinforced byirradiation in a wavelength range from 100 nm to 1000 nm, in particularbetween 350 nm and 800 nm. In this way, the machining of the depositionlayer can be done especially homogeneously. It is furthermoreadvantageous, by means of targeted doping of the deposition layer, tovary the porosity and thus the permeability of the porous silicon. Forinstance, p-doping is used to create mesoporous pores, while an n-dopingcan be used to create etching openings with a diameter ranging from afew nanometers up to several micrometers.

[0017] It has also proved advantageous to force the permeability of thedeposition layer by means of an also retroactively employed modifiedstain-etch operation, in which a mixture of hydrofluoric acid, nitricacid and water is employed. The porosity and depth of the etching can beadjusted via an adjustment of the mixture proportions and exposuretimes.

[0018] It has also proved especially advantageous to create thepermeability of the deposition layer by means of a galvanic process, inwhich a metal layer is applied to the region of the covering that is notto be changed. Simultaneously, the metal layer takes on the function ofa masking layer and need not necessarily be removed before the latersealing layer is applied. It comprises a metal that is nobler thansilicon, in particular such noble metals as platinum and gold. Theporosity of the porous (poly)silicon created during the galvanic processcan be varied as a function of a current density and the electrolytecomposition, and in particular via the area ratio of metal to silicon,since the latter represents the galvanic element, that is, the currentsource.

[0019] It has also proved advantageous that support elements areprovided on the underside of the deposition layer, which establish amechanically stable connection between the foundation wafer and thecovering. If the individual support elements or support struts arespaced apart from one another by from several micrometers to severaltens of micrometers, then on the one hand excessive sagging of the coverplate upon subjection to an overload is prevented, and on the other, theoverall stability is increased substantially.

[0020] It has furthermore proved advantageous to structure the sealinglayer as well by means of a masked etching process. The etching processused can also include structuring the deposition layer and optionallyeven further an upper layer of the foundation wafer, in particular ofepipolysilicon.

[0021] In a further advantageous feature of the method, the pressureinside the sensor chamber can be adjusted by way of the pressureconditions that prevail during the deposition of the sealing layer. Theprocess pressure prevailing during the deposition of the sealing layerwill automatically become established in the sensor chamber as well andbe sealed in there, while the sealing layer is growing. Depositionprocesses that can be considered for the sealing layer are sputteringprocesses (for metal layers) or PECVD processes (for SiN, SiO, SiC,etc.). If the enclosed pressure should not prove identical to thedeposition pressure, additional options still exist. Advantageously, tothat end, before or during the deposition the sensor is subjected to aninert gas, in particular helium, at a predetermined temperature that isintroduced additionally into the deposition chamber. Because of thepermeability of the deposition layer, a delayed pressure equalizationcan occur, and the fundamental diffusion processes can be ascertainedempirically. Since the suppression of the pressure equalization isachieved by applying the sealing layer at layer thicknesses of even onlya few micrometers or less, the sealing can be done within relativelyshort times.

[0022] Via the aforementioned method steps of the invention, capacitivepressure sensors can also be made in an especially simple way. As acommon feature, such pressure sensors have a differential capacitorarray, which is joined directly or via a coupling element to thecovering, so that sagging of the covering causes a change in thecapacitances of the differential capacitor array, and this change inturn serves as a measurement variable.

[0023] Further preferred features of the invention will become apparentfrom the other characteristics recited in the dependent claims.

DRAWINGS

[0024] The invention will be described in further detail below in termsof exemplary embodiments in conjunction with the associated drawings.Shown are:

[0025] FIGS. 1-13, the production method according to the invention inone exemplary embodiment;

[0026] FIGS. 14-16, an alternative production method, beginning afterthe deposition of a sealing layer in accordance with FIG. 8;

[0027]FIGS. 17 and 18, a further feature of the production method forcreating a surface-micromechanical capacitive pressure sensor with atorsion rocker;

[0028]FIGS. 19 and 20, a further alternative embodiment of thecapacitive pressure sensor;

[0029] FIGS. 21-23, an alternative procedure for varying thepermeability of the deposition layer by means of an electrochemicaletching operation;

[0030] FIGS. 24-27, alternative masking structures of metal layers thatcan be used in a galvanic process for adjusting the permeability of thedeposition layer;

[0031]FIG. 28, a covering with support elements;

[0032]FIG. 29, a covering with support elements and etching openings;

[0033]FIG. 30, a covering with support elements and a contacting regionfor bond pads;

[0034]FIG. 31, an alternative embodiment of the support elements;

[0035] FIGS. 32-34, a further feature of the production method forproducing a metal seal and a metal contact pad; and

[0036]FIG. 34′, an alternative embodiment with a dielectric as the sealand with a metal contact pad.

[0037] FIGS. 1-13 illustrate the production method according to theinvention for sensors, such as acceleration sensors or rotation ratesensors, and in particular also capacitive pressure sensors. The methodsteps sketched in FIGS. 1-4 are already known from DE 195 37 814 A1 andwill therefore be described below only briefly.

[0038] By definition, a subdivision of the sensor has already been madeinto a foundation wafer 11 and a thin-film sensor cap in the form of acovering 13. The foundation wafer 11 includes all the componentsnecessary for the function of the sensor, and in particular includescontact regions to be described in further detail hereinafter, as wellas micromechanical structures and electrodes. The covering 13 bydefinition extends from a deposition layer to and including a sealinglayer and serves the purpose of hermetically sealing a sensor chamber,in which the micromechanical structures are located.

[0039] Onto a silicon substrate 10, an insulation layer 12, whichencloses a conductive layer 14, is applied. Structuring of the twolayers 12 and 14 can be done by known method steps used in semiconductortechnology, such as lithography and etching processes and subsequentetching steps. A polycrystalline silicon layer 16 of desired layerthickness is also applied, which covers the insulation layer 12. Thesilicon layer 16 typically comprises epipolysilicon, while theconductive layer 14 is shaped of an optionally very highly dopedpolysilicon (FIG. 1).

[0040] By application of a masking layer 18, a region 20 is defined inwhich in later method steps the micromechanical structure is to becreated. First, the region is deepened (recess 20; FIG. 2) by apre-etching step.

[0041] In an ensuing lithography step (FIG. 3), the thus-prestructuredfoundation wafer 11 is coated in the recess 20 with a mask 22 ofphotoresist, which in advance defines the sensor structures to be made,such as capacitive comb structures, springs, stops, electrode faces, andperforations in a seismic mass. What is essential here is that theactual sensor structures be made with an adequately great spacing fromthe edges of the previously made recess 20, because there the precisionof lithography and resolution are otherwise impaired by the differencesin topography.

[0042] As can be seen from FIG. 4, in the regions not covered by thephotoresist 22, trenches 24, which extend as far as the insulation layer12, are made by known, suitable etching processes, such as in the mannerdescribed in DE 42 41 045. In this way, individual structures 26 areinsulated on the silicon layer 16. The design of such structures26—since it is known—will not be addressed in further detail inconjunction with the present description.

[0043] Next, sacrificial layer etching is done via the trenches 24 inthe region of the insulation layer 12, and a void 26 is created (FIG.5). The void 26 and the trenches 24 together form a sensor chamber 28,in which the structures 26 are accommodated. The sacrificial layeretching can be done for instance via an HF vapor etching process, or drysacrificial layer etching with silicon as the sacrificial layer inconjunction with a modified layer system. The resist mask 22 was removedbefore the conclusion of the sacrificial layer etching. In the case ofdry sacrificial layer etching with silicon as the sacrificial layer, themasking layer 18 is removed after the conclusion of the sacrificiallayer etching.

[0044] The entire structure is filled, in the method step sketched inFIG. 6, with a silicon-based oxide 30, in particular a CVD oxide or aporous oxide. The preferably highly porous oxide 30 must be removedagain at an extremely high etching rate using media that containhydrofluoric acid. The deposition conditions for the oxide 30 shouldaccordingly be selected such that a low-grade oxide 30 of high porosityis created. These are at the same time the conditions of depositionunder which deposition is done at the highest possible rate, which hasthe advantage of short oxide deposition times. The parameters fordesigning the deposition conditions of such low-grade oxides of highporosity are known from the prior art. For instance by means of highplasma capacities, the desired oxide film 30 can be deposited during thedeposition, at high process pressures and low substrate temperatures(such as 200° C. to 300° C.).

[0045] The next method step includes a re-thinning of the oxide 30 tothe height of the silicon layer 16 (FIG. 7), which can be done forinstance by grinding (CMP or Chemo-Mechanical Polishing) or a well-knownback-etching method. In the exemplary embodiment, the oxide 30 isremoved down to the height of the layer 16.

[0046] Next, a deposition layer 32, in particular of polysilicon, isdeposited as a deposition layer over the entire surface (FIG. 8). Toavoid compaction of the highly porous oxide 30, it is advantageous tokeep the temperatures as low as possible during this deposition. At theusual temperatures for polysilicon deposition of 600° C. to 650° C., forinstance, no significant compaction of the oxide 30 yet occurs. As analternative, it is possible to perform a brief heating of the foundationwafer 11 to temperatures of over 1000° C., without compacting the oxide30, if this heating is done only briefly (for instance for less than 10minutes). The tempering of the deposition layer 32 can advantageously bedone with the aid of an RTP (Rapid Thermal Processing) or RTA (RapidThermal Annealing) reactor. If epipolysilicon is deposited as thedeposition layer 32, then this deposition can be performed in anepitaxial reactor at a high deposition rate of up to 1 μm/min, in orderto keep the deposition times short. By means of such a brief, hightemperature treatment of the polysilicon, it can be attained for examplethat the layer 32 has a very high intrinsic tensile stress of over 200MPa/μm up to 1 GPa/μm of layer thickness, for example, which is quiteadvantageous for applying the layer 32 as a covering 13. Because of thishigh tensile stress, the layer 32 can withstand even a high pressureapplied from outside without bulging inward severely.

[0047] In an ensuing optional process step, the deposition layer 32 canbe structured, for instance by etching (FIG. 9). In the final analysis,then only the regions of the foundation wafer 11 among which themicromechanical structures 26 are later to be located inside the sensorchamber 28 will be covered by the deposition layer 32. It is alsopossible to use the covering 13 unstructured. In that case, the covering13 covers the entire wafer surface. The advantage then is that a planarsurface of the wafer is preserved.

[0048] By selective etching of the oxide 30 underneath the depositionlayer 32, it is possible to expose the structures 26 again (FIG. 10). Tothat end, the deposition layer 32 must necessarily be permeable to theetching medium used and to the products created during the reaction. Forinstance, the polysilicon of the deposition layer 32 can be provided forthis purpose with small etching openings by targeted etching, in amanner to be described in further detail later herein. Depending on thetype of etching process used, etching openings with diameters of 0.1 μmto 5 μm can be created. As an alternative, the deposition layer 32 cancomprise a permeable polysilicon or a polysilicon that is made permeableretroactively by porosification, which allows the etching medium orreaction products to pass through. In conjunction with hydrofluoric acidvapor etching processes, this variant has proved especiallyadvantageous, since the hydrofluoric acid vapor can penetratepolysilicon especially easily, and diffusion events for the speciesinvolved through the polysilicon can be speeded up, especially atelevated temperatures. The in-situ permeability of the polysilicon isespecially high at low dopant concentrations, as long as a long-lastinghigh-temperature treatment (tempering) is avoided. It is also possibleto employ a combination of HF liquid-phase etching and HF vapor etchingto remove the sacrificial layer. In the wet etching phase (HF solution),the majority of the oxide 30 is first removed. In the ensuing vaporetching, the remainder of the oxide 30 is removed without the risk ofsticking, resulting in an overall faster process.

[0049] After the conclusion of the sacrificial layer etching process,the deposition layer 32 is hermetically closed by deposition of asealing layer 34 (FIG. 11). It has proved especially advantageous toapply a silicon nitride layer as the sealing layer 34, because that canbe deposited at even relatively low temperatures of 300° C. to 400° C.in PECVD processes. As an alternative, the sealing can also be done bydepositing metals, such as aluminum, for which sputtering processes areparticularly well suited.

[0050] Depending on the material chosen for the sealing layer 34, astructuring of the covering 13 can then be done, for instance with theaid of photolithographic processes. If the chosen material is a metal(FIG. 12), then by means of these structurings the contact pads 36 cansimultaneously be formed. In the case of a dielectric material, thesealing layer 34 must be removed completely outside the capping region,and the contact pads 36 are applied (FIG. 13) in a separate method stepto the silicon layer 16 or the deposition layer 32 (if the latter isused unstructured over its full surface). The chosen layer thickness ofthe sealing layer 34 depends essentially on the permeability, porosity,and diameter of the etching openings of the deposition layer 32, and itis understood that a thicker sealing layer 34 is required for closingthe etching openings.

[0051] FIGS. 14-16 show an alternative process course, beginning withthe deposition of the deposition layer 32 as shown in FIG. 8. First, inthe manner already explained, the oxide 30 is etched away through thedeposition layer 32 (FIG. 14). Next, by means of the sealing layer 34,the sensor chamber 28 is hermetically sealed (FIG. 15). Once again, thelayer thickness must be selected to suit the properties of thedeposition layer 32, which are adjusted for the sake of the passage ofthe etching medium or reaction products. Next, by a masked etchingprocess shown in FIG. 16, both the deposition layer 32 and the sealinglayer 34 are removed, and the contact pads are applied to the siliconlayer 16 in a known manner. Overall, this makes it possible to dispensewith one mask plane, since the two layers 32, 34 are structured with oneand the same mask. In the case where permeable polysilicon is used asthe deposition layer 32, an adequate resistance must be assured afterthe structuring to prevent a diffusion of gases via the peripheralregions. In practice, it has been found adequate if the permeablepolysilicon layer overlaps the epipolysilicon layer 16 by at least 50μm.

[0052] In a further, already mentioned advantageous embodiment, thestructuring of the deposition layer 32 and the cover layer 34 is omitteduntil or until just before the deposition and structuring of the metalpads 36 (see FIGS. 32-34, 34′). Not until directly before the depositionof the metallizing 36 is etching of the sealing layer 34 performed, inthe immediate region of the bond pads and around them, in order to makethe bond regions free of insulating layers. In an especiallyadvantageous embodiment, the metal layer 36 for the bond padssimultaneously acts as a sealing layer 34. In this latter case, thesealing layer 34, after being deposited over the full surface, must beopened again around the bond pads, to enable through-etching of thedeposition layer 32 there and thus the creation of electricallyinsulated contact pads 36.

[0053] In a further variant, the epipolysilicon layer 16 is initiallynot yet etched through in the region of the contact pads 36, but insteadis still present over the full surface. In the production of the sensorstructures 26 by anisotropic deep etching, the bond regions areaccordingly initially not included. Not until the contact pads 36 areetched free, or in other words the deposition layer 32 and the sealinglayer 34, if the latter is not identical with the metallizing layer ofthe contact pads 36, are etched through is the epipolysilicon layer 16etched through, down to the buried oxide 12, around the bond pads. Thesame deep etching process can be employed for both the silicon layer 32and for the epipolysilicon layer 16. Accordingly a “double-trench”process takes place, in which a first deep trench is made for the sensorstructures 26 themselves, and a second deep trench for the bond padregions is made later in the process sequence. In both variants, theelectrical connection of the sensor structures 26 is effected via thecontact pads 36 through the deposition layer 32 and the layer 16. Thedeposition layer 32 must accordingly have adequate electricalconductivity to enable large-area electrical contact, but in practicethis is not a problem, given adequate doping, even if the depositionlayers 32 are relatively thick. The aforementioned variants can belearned for the sake of illustration from FIGS. 32-34, in which a metalis used as the seal and bond pad, and FIG. 34′, in which a dielectric isused as a seal and metal is used as the bond pad.

[0054] The process steps, sketched in FIGS. 15 and 11, in which thesealing layer 34 is applied can be used especially advantageously foradjusting a desired internal pressure inside the sensor chamber 28. Theadjustable pressure range includes pressures from a few microbars up toatmospheric pressure. In contrast to the conventional methods, becauseof the very rapid sealing the tolerance in the pressure adjustment canbe kept relatively low. For adjusting the pressure, the procedure can beas follows:

[0055] In an embodiment with a permeable deposition layer 32 (one thatis in-situ permeable, or is retroactively made porous to createpermeability), after the foundation wafer 11, by then fully processed upto the point of the application of the sealing layer 34, is insertedinto a process chamber for deposition of the sealing layer 34, heatingis first done to a temperature between 300° C. and 450° C.; at the sametime, instead of process gases, an inert gas is delivered to the processchamber at a given pressure. As the inert gas, helium is for instancesuitable, since it can diffuse especially quickly through the depositionlayer 32 that is either in-situ permeable or made permeable byporosification, so that in the sensor a rapid establishment ofequilibrium (internal pressure equals outer pressure) is possible. Onlyafter that are the process gases required for the deposition of thesealing layer 34 supplied and the deposition plasma ignited. Ifpermeable polysilicon (in-situ permeable, or made permeable byporosification is used, then only a few seconds elapse in the timebetween the leaving behind of the desired concluding pressure in theprocess chamber and the deposition of an adequately thick sealing layer34.

[0056] For a known layer thickness and permeability of the depositionlayer 32, the pressure change to be expected in this time can becalculated, so that suitable precautions can be planned for. It is alsopossible initially to leave the deposition pressure at the desiredenclosed pressure for the capping, and already to start the depositionprocess early while still in the presence of the inert gas. Only afterthe initiation of the deposition process is the chamber pressurereadjusted to the pressure range that is actually optimal for thedeposition. As a result, while the deposition process proceedsnon-optimally for a few seconds and reaches its most favorable operatingrange only after the pressure adaptation has been made, still proceedingin this way means that the time between leaving the capping pressure andthe completed hermetic sealing of the sensor element is shortened. Forexperimentally determining the enclosed pressure and monitoring themethod, the diaphragm bulging can be assessed, for instance byinterferometry, or quality parameters of the enclosed structures 26 canbe ascertained by resonant excitation. Quality control for the method isalso easily possible.

[0057] It has proved to be especially favorable for this process courseto be used to produce surface-micromechanical capacitive pressuresensors. FIGS. 17 and 18, on the one hand, and 19 and 20, on the other,accordingly show two possible embodiments of such pressure sensors.First, as shown in FIGS. 17 and 19, a layer 32 that is permeable or isretroactively made permeable by etching is deposited—optionally instructured form—above the sensor chamber 28 that is filled with theoxide 30. Subsequent etching out of the oxide 30 and sealing with thesealing layer 34 lead to the application forms shown in FIGS. 18 and 20.

[0058] In the first case, a torsion rocker 39 is implemented in thepressure sensor; this rocker is connected to the covering 13 via acoupling element 42. A seismic mass 38 is suspended—analogously to abeam scale—symmetrically via torsion springs 40 and centrally withregard to both sides. The mask 32 is perforated for the performance ofthe sacrificial layer etching; the perforation is not shown here. Afterthe deposition and planarization of the oxide 30, a hole is made in theoxide 30, somewhat outside the middle of the torsion rocker 39, using aphotographic technique, and the oxide 30 is etched. Above it, thedeposition layer 32 can for instance simply and advantageously bedeposited as a permeable polysilicon layer, and/or it can be madepermeable retroactively by etching processes, such as porosification; inthe “contact hole” previously placed in the oxide 30, the silicon of therocker 39 can be contacted directly both mechanically and electrically.If an electrical insulation of the sensor diaphragm from the torsionrocker 39 is later desired, for instance for the sake of electricalshielding from the environment, then before the polysilicon isdeposited, an insulating layer can be deposited that is not attacked bythe HF vapor chemistry employed afterward for sacrificial oxide etching.An example of something suitable for this is a layer of amorphoussilicon carbide, which is resistant both to media that containhydrofluoric acid and to HF vapor. This layer can be structured afterthe conformal deposition above the contact hole in the oxide either by amasked etching process, such that only the coupling element 42 remains,or can be machined in such a way that after a grinding process, thecoupling element 42 remains, enclosed by the oxide 30. It is understoodthat in that case the order of the process steps can also be reversed;that is, the coupling element 42 (for instance of amorphous siliconcarbide) is applied first, and then the filler oxide is deposited andplanarized, and then the entire covering 13 is deposited and planarized,taking into account the pressure adjustment process parameters setbeforehand.

[0059] By the two production variants for the coupling element 42—thatis, simple polysilicon deposition with mechanical connection via thepolysilicon, which it is understood fills up the contact hole in theoxide 30 and thus establishes the nonpositive engagement with the rocker39, or the explicit deposition and production of an electricallyinsulated coupling element 42 by means of an addition layer—a mechanicalconnection is created between the covering 13 and the torsion rocker 39.

[0060] Because of the bending form of the covering 13 that is subjectedto pressure, it is advantageous to place the coupling element 42 betweenthe torsion axis and the center of the cover plate, or accordingly forexample—as seen in FIGS. 17, 18—to the right of the torsion axis of therocker 39 and to the left of the middle of the diaphragm. If pressure isexerted on the diaphragm, a bending line in the form of a double-Sensues, which presses the right-hand half of the rocker 39 downward, andthe left-hand half of the rocker 39 accordingly moves upward. If thereare two counterelectrodes (structured from the conductive layer 14)underneath the rocker 39, then the change in capacitance can beprocessed as a differential capacitance by means of suitable evaluationelectronics. The electrical wiring of the sensor component is done inthe bottom plane, through the conductive layer 14 buried there. Such asensor, with the layout described, has an advantageously low temperaturedrift, because of its symmetry and the capacitive assessment by thedifferential capacitor array, making it possible to dispense with anexpensive calibration and temperature compensation.

[0061] If the explicit differential capacitor array is dispensed, then asimpler process and design in accordance with FIGS. 19 and 20 can beconsidered. The seismic mass 44 is joined directly to the depositionlayer 32. Also, in the way described above, a reference pressure,preferably by means of helium gas, can be enclosed in the sensor chamber28, and the structure can be hermetically sealed by sealing of thesealing layer 34.

[0062] If pressure is exerted on the structure, the mass 44 is presseddownward, thus decreasing the spacing from the layer 14 lying below,which functions as a counterelectrode, and a change in capacitanceaccordingly ensues. The electrical connection and the embodiment of thecounterpart electrode can in turn be accomplished via the layer 14 andextended to the outside. One thus obtains a simple, robust, capacitivepressure sensor by means of surface micromechanics. The evaluationelectronics developed in standard form for acceleration sensors cancontinue to be used, if a differential capacitor array is embodied by afixed-value capacitor connected externally to the measuring capacitor.

[0063] The permeability of the deposition layer 32 for the etchingmedium and the resultant reaction products can also be forcedretroactively after deposition of the layer 32. A first method of thiskind is sketched in FIGS. 21-23, in which an electrochemical etchingoperation is primarily used for converting silicon into (permeable)porous silicon. First—as already described—the procedure is the same upto and including the deposition of the deposition layer 32. Next, asuitable masking layer 46 is applied (FIG. 21), and in a known manner,such as by an additional lithography step, structuring is performed, sothat a region 48 in which the properties of the layer 32 are to bevaried or modified is made accessible (FIG. 22).

[0064] The actual electrochemical etching operation is performed in thepresence of an HF electrolyte, such as a mixture of hydrofluoric acidand ethanol, and leads to the formation of porous structures or etchingopenings in the regions 48 of the layer 32 that are exposed to theelectrolyte.

[0065] It has proved especially advantageous in the electrochemicaletching operations of the type described to perform an irradiation ofthe surface in addition, in a wavelength range from 100 nm to 1000 nm,and in particular 350 nm to 800 nm, since the homogeneity of the processis improved thereby. An electrical connection by application of ananodic potential can be done on the one hand via the top side of thelayer 32 and on the other from the epipolysilicon layer 16 or thefoundation wafer 11 (back-side contact) via the underside of the layer32. The large-area back-side contact via the foundation wafer 11 has theadvantage that with it, a better-defined, more-homogeneous distributionof current density of the anodizing current is achieved, since thecurrent has to overcome a maximum of only the thickness of thefoundation wafer 11 in order to reach the region 48 to be treated.Expediently, a high n-doping of the layers of the foundation wafer11—above all, the wafer underside of the substrate (10)—is provided(n⁺⁺), which becomes possible especially simply by POCL deposition andensuing forcing of phosphorous into the silicon, but also by ionimplantation of phosphorous, arsenic or antimony. The n⁺⁺ doping of theback side of the foundation wafer 11 reduces the Schottky barrier thatis present in the electrolyte/silicon contact region. Suitably adapteddoping of the layer 32 in the region 48 that is to be varied can be usedto control the process. It has been demonstrated for instance thatp-doping leads to the formation of mesoporous pores, while n-dopingleads to etching openings ranging from a few tens of nanometers tomicrometers.

[0066] Alternatively to the electrochemical etching operation, theprocedure can be as shown in FIGS. 24-27 for exemplary embodiments.First, the still inadequately permeable layer 32 is applied, and next,using known masking methods, a metal layer is deposited and structured.In the ensuing galvanic production of porous polysilicon in the region48, the metal thus simultaneously takes on the function both of maskingthe silicon surface of the layer 32 in the regions that are not to beelectrochemically anodized, and of a cathode in the galvanicsilicon/electrolyte/metal cell. The processes that lead to the formationof the porous polysilicon can be controlled via the composition of theHF electrolyte and via the incident current density in this galvaniccell. The current densities are dependent on the ratio of area of metalto silicon. The larger the metal area, the higher the current density.Typical metal to silicon area ratios are between 10 and 20 to 1. Theadvantage of this technique is that electrical contacting of the waferis not necessary.

[0067] To achieve these ratios, parts of the metal face of the region48, which is to be made porous, of the covering 13 can be covered with agrid. Care should be taken that the width of the metal tracks is greaterthan the thickness of the layer 32 to be etched, because otherwiseexcessive underetching and detachment of the metal could occur. Aselection among possible embodiments can be learned from the plan viewsand sectional views of FIGS. 24-27.

[0068] It is also conceivable, by a modified stain-etch operation, totreat the regions 48 that are to be made porous with a mixture ofhydrofluoric acid, nitric acid and water. All the other regions must beprotected with a suitable masking layer, for instance of siliconnitride. By way of the composition, and in particular the nitric acidconcentration, and the exposure times, the porosity and layer thicknessof the modified porous silicon region can be controlled. Moreover, thereis an empirically detectable influence of dopants, making it possible tocontrol the process that creates the porosity.

[0069] A further alternative embodiment of the thin-film sensor cap, inwhich support elements 50 are present on the underside of the depositionlayer 32, can be seen in FIGS. 28-31. Up to the deposition of the oxide30, as shown in FIG. 6, the method described at the outset can beemployed. However, a complete planarization of the oxide film 30 down tothe height of the epipolysilicon layer 16 is dispensed with. Instead, astructured removal of material is done, in which the oxide 30 is removedfrom those regions that are later to form the support elements 50. Theseregions are logically located above the regions of the epipolysiliconlayer 16, which are not supposed to be further attacked in the ensuingetching process.

[0070] The individual support elements 50 are typically encompassingsupport struts or support columns, which thus define the sensor chamber28 that is covered by the covering 13. The necessary micromechanicalstructures 26 are located inside the sensor chamber 28. In accordancewith FIG. 28, slight spacings of the support elements 50 and thus slightclamping widths of the covering 13 can be achieved. Clamping widthsbelow 10 μm are thus feasible. However, this also means reduced saggingupon subjection to an overpressure, and the spacing of the covering 13and the sensor element can be reduced so much that it is impossible tolift the sensor structure 26 out in the event of a mechanical overload.Since the bond frames necessary in the conventional sensors can bedrastically reduced in size, a major reduction in surface areaadditional ensues, so that more than twice as many acceleration sensorscan be processed on the same foundation wafer 11. FIG. 31 in thisrespect shows a further advantageous embodiment with T-shaped supportelements 50, which lead to especially stable structures.

[0071] For the case where instead of using a permeable polysilicon asthe deposition layer 32, etching openings 52 by way of which thesacrificial oxide etching takes place are to be made retroactively, thedesign shown in FIG. 2 has proved advantageous. The etching openings 52are disposed here such that upon the deposition of the sealing layer 34,at most the structural elements 53 of the sensor that are notfundamental to its function are exposed to the deposition plasma. Thesestructural elements 53 that are not fundamental to the function are infact precisely those elements that are connected to the support elements50. After the sacrificial etching, it is optionally possible via theetching openings 52 to deposit suitable anti-adhesion layers in theregion of the structures 26 as well.

[0072] A sensor in accordance with FIG. 30 can be made with the aid ofthe process steps described above. Along with the possibility, describedin conjunction with FIGS. 15 and 16, of simultaneously structuring boththe deposition layer 32 and the layer 34, or of initially notstructuring the deposition layer 32, and initially leaving it over thefull surface and only at the end etching contact pads 36 that areelectrically insulated from the system, it is also possible to machinethe epipolysilicon layer 16 located beneath, so that the opening 54, byway of which contacting can later be done, can be created in one processstep. In electrochemical etching, contacting in turn occurs via the backside of the wafer. The layers are then electrically connected via thesupport elements 50, so that a preferably high permeability isestablished in the region of the support elements 50 by the formation ofporous silicon. This electrical contacting of the layer 32 can also bedone next to the sensor region 28, toward the substrate 10.

1. A sensor with at least one silicon-based micromechanical structure,which is integrated with a sensor chamber of a foundation wafer, andwith at least one covering that covers the foundation wafer in theregion of the sensor chamber, characterized in that the covering (13)comprises a first layer (32) (deposition layer) that is permeable to anetching medium and the reaction products, and a hermetically sealingsecond layer (34) (sealing layer) located above it.
 2. The sensor ofclaim 1, characterized in that the deposition layer (32) is permeable inthe region of the sensor chamber (28) to the etching medium and thereaction products.
 3. The sensor of claim 1, characterized in that thedeposition layer (32) has etching openings (52) in the region of thesensor chamber (28).
 4. The sensor of claim 3, characterized in that theetching openings (52) have a diameter of 0.1 to 5 μm.
 5. The sensor ofone of the foregoing claims, characterized in that the deposition layer(32) has support elements (50) on its underside that create a connectionbetween the foundation wafer (11) and the covering (13).
 6. The sensorof claim 5, characterized in that the support elements (50) are supportstruts, disposed parallel to the structure (26), that encompass thesensor chamber (28) and whose spacing is in a range from 5 to 1000 μm.7. The sensor of claim 6, characterized in that the support elements(50) are encompassing support struts or support columns.
 8. The sensorof claims 3 and 5, characterized in that the etching openings (52) aredisposed in the region of the support elements (50) such that a directexposure of the structure (26) to the material forming the sealing layer(34) is avoided upon the deposition of the sealing layer, and thestability of the deposition layer (32) is preserved.
 9. The sensor ofone of the foregoing claims, characterized in that the deposition layer(32) is of polysilicon.
 10. The sensor of claim 9, characterized in thatthe deposition layer (32) is subdivided into regions of high, low orabsent porosity, and the regions of high porosity are located maximallyabove the sensor chambers (28).
 11. The sensor of claim 10,characterized in that a metal masking layer, in particular comprising ametal that is nobler than silicon, is applied to the deposition layer(32) in regions of low or absent porosity.
 12. The sensor of one of theforegoing claims, characterized in that the sealing layer (34) is aninsulator, in particular of silicon nitride or silicon oxide.
 13. Thesensor of one of claims 1-11, characterized in that the sealing layer(34) is of metal, in particular aluminum.
 14. The sensor of claim 13,characterized in that the structure (26) is covered in the sensorchamber (28) with an anti-adhesion layer.
 15. The sensor of one of theforegoing claims, characterized in that the sensor is an accelerationsensor or rotation rate sensor.
 16. The sensor of one of the foregoingclaims, characterized in that the sensor is capacitive pressure sensor.17. The sensor of claim 16, characterized in that in the capacitivepressure sensor, a seismic mass (38, 44) is bonded to the covering (13)either directly or via a coupling element (42).
 18. A method forproducing a sensor with at least one silicon-based micromechanicalstructure, which is integrated with a sensor chamber of a foundationwafer, and a covering that covers the foundation wafer at least in theregion of the sensor chamber, characterized in that (a) at least thesensor chamber (28) present in the foundation wafer (11) after theestablishment of the structure (26) is filled with an oxide (30), inparticular CVD oxide or porous oxide; (b) the sensor chamber (28) iscovered by a first layer (32) (deposition layer), in particular ofpolysilicon, that is transparent to an etching medium and the reactionproducts or is retroactively made transparent; (c) the oxide (30) in thesensor chamber (28) is removed through the deposition layer (32) withthe etching medium; and (d) next, a second layer (34) (sealing layer),in particular of metal or an insulator, is applied to the depositionlayer (32) and hermetically seals off the sensor chamber (28).
 19. Themethod of claim 17, characterized in that before the application of thedeposition layer (32), the oxide (30), in regions outside the sensorchamber (28), is removed by etching or grinding, in particular by CMPgrinding (planarization of the surface of the foundation wafer).
 20. Themethod of claim 17, characterized in that before the application of thedeposition layer (32), the oxide (30) is structured, in regions outsidethe sensor chamber (28), by masked etching (structuring of the surfaceof the foundation wafer).
 21. The method of claim 20, characterized inthat the oxide (30) is removed in regions in which a support element(50) is provided, on the underside of the cap region, as a bindingmember between the foundation wafer (11) and the covering (13).
 22. Themethod of one of claims 18-21, characterized in that etching openingswith a diameter of 0.1 to 5 m are made in the deposition layer (32) byetching, in particular by masked plasma etching.
 23. The method of oneof claims 18-22, characterized in that the permeability of thedeposition layer (32) is forced by means of an electrochemical etchingoperation, in which a mixture of hydrofluoric acid and ethanol forinstance serves as the electrolyte.
 24. The method of claim 23,characterized in that the top side of the deposition layer (32) iscovered with a masking layer (46), which is removed in the regions (48)that are to be made porous.
 25. The method of one of claims 23 or 24,characterized in that an electrical connection is made by theapplication of an anodic potential to a top side of the deposition layer(32).
 26. The method of one of claims 23 or 24, characterized in thatthe electrical connection is made by application of an anodic potentialto an underside of the deposition layer (32), via a lower-lying layer ofthe foundation wafer (11) or via the foundation wafer (11) itself. 27.The method of one of claims 23-26, characterized in that thepermeability is varied via doping of the deposition layer (32).
 28. Themethod of claim 27, characterized in that a p-doping of the depositionlayer (32) is utilized to create mesoporous pores.
 29. The method ofclaim 27, characterized in that an n-doping of the deposition layer (32)is utilized to create etching openings (52) having a diameter rangingfrom a few tens of nanometers to a maximum of 10 μm.
 30. The method ofone of claims 18-22, characterized in that the permeability of thedeposition layer (32) is forced by means of a masked stain-etchoperation.
 31. The method of claim 30, characterized in that thestain-etch operation is effected by means of a mixture of hydrofluoricacid, nitric acid and water, and the porosity and etching depth of theporous layer into the deposition layer (32) are adjusted via the mixtureproportions and the exposure times.
 32. The method of one of claims18-22, characterized in that the permeability of the deposition layer(32) is achieved by means of a galvanic process, in that a metal layeris applied in the region that is not to be changed (masking), and thatduring the ensuing galvanic process, etching is done at a boundary facebetween the HF electrolyte and the unmasked deposition layer (32), as afunction of a current density and/or an area ratio of metal to siliconand/or as a function of an electrolyte composition.
 33. The method ofone of claims 23, 30 or 32, characterized in that additionally duringthe etching operation, an irradiation takes place in a wavelength rangefrom 100 nm to 1000 nm, and preferably between 350 nm and 800 nm. 34.The method of one of claims 18-33, characterized in that the sealinglayer (34) is structured by means of a masked etching process.
 35. Themethod of claim 34, characterized in that the masked etching processincludes structuring of the deposition layer (32).
 36. The method ofclaim 35, characterized in that the masked etching process additionallyincludes structuring of an upper layer of the foundation wafer (11), inparticular of epipolysilicon.
 37. The method of one of claims 18-36,characterized in that the pressure inside the sensor chamber (28) isadjusted via the pressure conditions during the deposition of thesealing layer (34).
 38. The method of claim 37, characterized in thatbefore the deposition of the sealing layer (34), the pressure inside thesensor chamber (28) is adjusted by subjecting it to an inert gas, inparticular helium, at a predetermined temperature.
 39. The method ofclaim 38, characterized in that the deposition of the sealing layer (34)already begins in an atmosphere that contains inert gas, and the optimaloperating parameters for a deposition plasma are adjusted gradually.